Nuclear Quadrupole Resonance (NQR) is a well-known spectrographic technique that is used to detect and identify molecular structures by the characteristic NQR of atomic species contained within. Certain atoms' nuclei have the characteristic of absorbing RF energy when exposed to a frequency that causes its nucleus' spin axis to hop between several stable orientations. This is possible only if the particular nucleus has a non-symmetrical charge distribution that permits interaction with the atom's electron cloud non-symmetries. The complexity of these quasi-stable orientations typically leads to a series of closely spaced, narrow line width absorption lines. An example of such a nucleus is the common isotope Nitrogen-14.
This characteristic resonance has been used commercially to positively detect substances such as explosives contained within shipping containers. The method used is to sweep a local RF field through the frequencies of interest and, by using a bridge structure, measure the loading on the RF source as it passes through the resonances and use this information to identify the material under observation.
There are several technical difficulties with this method that limits its usefulness to very short range applications on the order of one meter, whereas it would be very useful to provide much greater range when dealing with materials such as explosives, i.e. tens of meters or further of standoff would be desirable.
Issues with the conventional NQR spectrographic techniques concern the ability to measure very small signals buried within very large ones, and the directionality of the frequencies used to stimulate the specimen under study.
There are two principal conventional NQR techniques. The first one relies on detecting a target's absorption of resonant radio frequency energy from an interrogator tuned to the specimen's resonant frequency. This is usually accomplished with a bridge configuration in which the voltage across a coil of wire acts as an exciter at the interrogating frequency and that voltage is nulled in a bridge circuit at a frequency that is near an expected resonance but not at it. When the exciter's frequency is tuned past a specimen's resonance, a signal appears in one arm of the bridge whose amplitude indicates the degree of absorption by the specimen. A problem with this approach is that the circuit impedance varies with frequency even if there is no resonance due to the reactive elements used in the exciter, and the RF impedance of the specimen may also vary as the frequency is swept during analysis. These effects conspire to limit the sensitivity of a spectrometer using this approach, a limitation that is typical of any spectrometric technique that uses the same frequency to interrogate as that to which the detector is sensitive. Examples of spectrometers for which this is not so are Raman and florescence spectroscopy, in which the illumination frequency is very different than the response of the specimen, so the illuminator can easily be filtered out and the detector can therefore be made very sensitive to the alternate frequency since it is not blinded by its interrogator.
The second major type of conventional NQR spectrometer is based on producing an echo from the sample. In this method the interrogator emits a pulse of resonant frequency energy toward a specimen under study, and then its receiver listens for immediate re-emission of the same frequency energy when the interrogator is turned off. This mode is less prone to blinding by the interrogator but is less sensitive over all because of three effects. First, the average energy available to excite the specimen is lowered by the duty cycle of the system. Second, its noise figure is worsened by the necessity to pass wide band RF pulses thereby increasing the level of thermal noise in the receiver. The latter effect is particularly bad since the resonances can have bandwidths of single Hertz while the millisecond time delay of the echo mandates a minimum bandwidth of several kilohertz, increasing the noise figure on the order of 30 dB. The third problem with the echo approach is the very high transmit/receive signal loss. The amount of RF energy stored up to be reemitted is quite small, so it is a weak signal with which to start.
All of the resonances of nuclei that are of interest occur below about 1 GHz. Nuclei that resonate toward the high end of this range may benefit from directional interrogators but many important species occur in the hundreds of KHz to a few 10's of MHz. Nitrogen 14 is particularly important to the detection and identification of hidden explosives and these are in the few MHz range and below. Antennas that operate in this range are either very large or will experience very poor directionality. Thus, even if a stand-off system is constructed that overcomes the sensitivity limitations previously described it will typically not be able to resolve its location in the field with sufficient accuracy to locate it.
Atmospheric and man-made noise in this frequency range is far above thermal limits so even if the system is ideally constructed its sensitivity may be further limited by ambient rather than system noise.
These factors combined yield a very short range of operation, and commercial systems therefore operate in near proximity to the target, on the order of 1 meter or less range. This range is entirely unsatisfactory for many applications such as locating hidden explosives that are connected to trip wires.
Therefore there is a need for an improved system and method for sensing a nuclear quadrupole resonance of a specimen.